1. Field of Invention
The invention relates generally to the field of pulping and bleaching lignocellulosic materials. More specifically, the present invention is directed to pulping and bleaching of lignocellulosic materials which includes biopulping and/or water extraction processes.
2. Description of the Prior Art
There are a number of processes that convert lignocellulosic materials to pulp. Pulp is the fibrous slurry that is fed to a paper machine to produce paper. Mechanical, chemical and hybrid methods dominate commercial pulping plants. About 25% of worldwide pulp production is mechanical pulp. It is a high-yield process but suffers from high energy costs and damage to the lignocellulosic fibers. This damage produces lower strength paper. These disadvantages (cost and quality) limit the number of applications for pulp.
Chemical pulp is the pulp produced by chemical pulping. The dominant chemical wood pulping process is the kraft process. In this process a digesting solution of sodium hydroxide and sodium sulfide is employed. The advantage of chemical pulp is reduced damage to the lignocellulosic fibers insofar as the chemical pulping operation permits a sufficient amount of the lignin constituent in the lignocellulosic materials to be dissolved so that the lignocellulosic fibers separate without significant mechanical action.
Recently, a means for improving pulping has been developed. That new development is the addition of a biopulping step. The production of pulp begins with lignocellulosic materials, such as wood chips. When a biopulping step is used, the lignocellulosic materials are ‘digested’ with one or more fungi types prior to mechanical or chemical pulping. The fungi soften the lignocellulosic materials by degrading or breaking lignin-carbohydrate complexes in the lignocellulosic materials.
A process that describes bioprocessing in detail is U.S. Pat. No. 6,402,887 whose disclosure is incorporated herein by reference. That patent describes a process of biopulping of industrial wood waste using fungi which selectively degrade lignin.
After biopulping, the wood chips are mechanically or chemically pulped into individual fibers. The fungi and the produced enzymes are destroyed during the thermomechanical pulping process. Due, in large part, to the biochemical action of the fungi, less energy is required to convert the chips to fibers. Some investigators claim energy savings of at least 30%. The easier conversion from chip to fiber means less damage to the fibers. The paper formed from these fibers is stronger.
Although a biopulping step reduces the energy costs associated with pulping, it does not address the absence of recovery of the full commercial value of lignocellulosic materials. Lignocellulosic materials comprise cellulose, lignin and hemicellulose. Conventional pulping operations recover the cellulose values in the form of fibers. The value provided by lignin, which is removed in the pulping operation, is recovered as energy, by its combustion.
That is, conventional pulping, whether or not including a biopulping step, does not address a major aspect of commercial exploitation of lignocellulosic materials. As stated above, there are three major components in lignocellulosic materials. The first is cellulose. The pulping operation yields fibers which are substantially the cellulose component. A second component is lignin, which is removed in the pulping operation. Indeed, biopulping involves fungal digestion of lignin. The third component, which is usually utilized for its energy value, along with the lignin, is hemicellulose.
Hemicellulose is a mixture of sugar and sugar acids, a major component of which are xylans. The difficulty in the prior art of isolating the product values of hemicellulose has limited the utility of the hemicellulose component in wood to the marginal energy value of that component. An acid pretreatment can be used to depolymerize the xylan to xylose and xylose oligomers. The acid would also catalyzes hydrolysis of acetyl groups (2-4.5% of the weight of the original wood) to acetic acid. If the wood is treated with hot water a low initial rate of acetic acid would be obtained. However, each acetic acid molecule formed would then act as an acid catalyst in a process referred to as autohydrolysis.
Additionally, there are some drawbacks to biopulping, such as a reduction in the brightness and opacity of the resulting fibers. Since the production of higher quality papers is desirable, use of biopulped fibers will require improvements in brightness and opacity. Research is underway to develop strategies to address these drawbacks. Preliminary bleaching studies with hydrogen peroxide and addition of calcium carbonate to improve both brightness and opacity have met with early success.
The present invention provides a method for producing pulp that addresses the above and other issues.